I use my phone on the charger a lot, in fact it almost ends up that way each night as I squeeze the last bit of use before having to plug it back in...

I use my phone a LOT, and I think the one downside is the heat that gets generated by doing this...

Still with a new battery being $10, does it really matter?

You should have no problem using the phone while plugged, but not sure how it works when you use it after it reaches 100%... People have said it reverts back to battery use, but when I start using it, I just disconnect it anyways... Since you can charge Li-Ions as often as you need, there's no point in leaving it plugged once you feel the need to use it...

It's not using the battery in any appreciable way while charging if the handset is off, and usually not even if it's on and active if the run-time current draw is sufficient to be supplied over USB. Most of the handsets I've worked with have intelligent PMICs that handle this kind of thing transparently.